Multilayer printed circuit board structure

ABSTRACT

A composite printed circuit board structure including multiple layers of graphite interleaved with layers of a dielectric material, such as a polytetrafluoroethylene (PTFE) and woven glass laminate. Some of the dielectric layers are copper clad, and at least some of the graphite layers are positioned in close proximity to the copper cladding layers, to provide good heat dissipation properties. The PTFE provides a desirably low dielectric constant and the graphite also provides good mechanical strength and a low or negative coefficient of thermal expansion, to permit matching of the coefficient with that of chip carriers used to mount components on the circuit board.

BACKGROUND OF THE INVENTION

This invention relates generally to printed circuit boards, and moreparticularly, to printed circuit boards on which microelectroniccomponents or chips are mounted. One desirable characteristic of printedcircuit boards used for this purpose is that they should be mechanicallystable under conditions of varying temperature. This is particularlyimportant in circuit boards used to support microelectronic componentshoused in chip carriers.

Chip carriers are usually fabricated from a ceramic material, such asaluminum oxide, and are produced in the form of a hermetically sealedpackage for each chip. Bonded leads are brought out from the chip to theedges of the chip carrier, and the carrier is then usually soldered, byits leads, directly to a circuit board. The principal advantage of thisstructure is a significantly higher circuit density. Also, the use ofshorter and more uniform lead lengths results in improved speed andimpedance characterisitics. Another consideration is that the use ofchip carriers substantially reduces the overall cost of a circuitpackage. Package size reductions as high as a five-to-one ratio can beobtained, compared with an equivalent dual in-line package construction.

A major drawback to the use of chip carriers is that the coefficient ofthermal expansion of aluminum oxide, the most commonly used chip carriermaterial, is approximately one-half the coefficient of thermal expansionfor glass/epoxy laminates typically used in the manufacture of circuitboards. When the resulting structure is exposed to any significant rangeof temperatures, the thermal cycling of the structure can crack solderedjoints and render the circuit inoperative. One solution to this problemis to use an intermediate member between the chip carrier and thecircuit board. The circuit board is sometimes referred to as a motherboard, and the intermediate member as a baby board. The intermediatemember may also take the form of a hybrid package on which the chipcarrier is mounted. Another technique is to use a compliant leadstructure between the chip carrier and the circuit board, although thisclearly increases the cost of the package and results in inherently longlead lengths.

Accordingly, an ideal circuit board should have a coefficient of thermalexpansion that closely matches that of chip carriers mounted on theboard. If there is a substantial mismatch in coefficients of thermalexpansion, the chip carrier may break loose from the board, or theelectrical connections may be damaged.

Another difficulty that has arisen as larger numbers of components aremounted on circuit boards, is that the heat produced by the componentsmust be dissipated in some manner, whether by conduction through thecircuit board or by radiative, convective, or forced-air cooling. Sincethe principal materials used in circuit boards are insulators, theboards themselves have traditionally played no significant role indissipating heat from the components that they support.

A third factor in the design of circuit boards is that they shouldideally be of a material with a relatively low dielectric constant, toenhance the board's ability to propagate signals over relatively longdistances.

Some materials, such as polytetrafluoroethylene (PTFE) have gooddielectric properties but an undesirably high coefficent of thermalexpansion. Kevlar (trademark of E.I. du Pont de Nemours & Co., Inc.) hasa negative coefficient of thermal expansion, and may be used to reducethe average coefficient of thermal expansion in a composite circuitboard structure. However, Kevlar is a poor thermal conductor, andtherefore does nothing to enhance the thermal conduction properties ofthe board.

U.S. Pat. No. 4,318,954 issued to Jensen, proposes the use of a singlethick layer of graphite reinforced with a resin, to adjust thecoefficient of thermal expansion of a circuit board. The techniquedisclosed in the patent is to use a large bulk of graphite, such thatthe composite expansion coefficient approaches that of the graphitealone. However, the Jensen patent does not provide any solution to theincreasing problem of heat dissipation.

It will be appreciated from the foregoing that there is an everincreasing need for a multilayer printed circuit board structure thataddresses these problems. Specifically, the ideal circuit boardstructure should have low dielectric properties, a low or negativethermal coefficient of thermal expansion, and good thermal conductionproperties to enhance heat conduction from devices mounted on the board.The present invention satisfies all of these needs.

SUMMARY OF THE INVENTION

The present invention resides in a multilayer printed circuit boardstructure in which multiple layers of graphite are employed both toreduce the coefficient of thermal expansion and to provide enhancedthermal conductivity, and multiple layers of a PTFE material are used toprovide the necessary dielectric properties. More specifically, thelayers of graphite are spaced symmetrically across the thickness of thecircuit board, to minimize the possibility of bowing of the board duringtemperature changes, and at least some of the layers of graphite arepositioned in close proximity to copper layers in the board, to provideenhanced thermal conduction from the mounted components.

The graphite layers in the structure of the invention take the form ofwoven sheets of fabric that have been impregnated with a bondingmaterial, such as an epoxy resin. An alternative preferred construction,in which high modulus carbon fiber material is used, takes the form ofalternative layers of unidirectional fibers, the fibers in each layerbeing more or less parallel to one another and the alternate layer fiberdirections being arranged to provide stiffness and strength along themajor axes of the structure of the invention. Each graphite sheet isseparated from adjacent copper or dielectric layers by a thin layer ofinsulating adhesive material. The adhesive serves both to bond the twolayers together and to electrically insulate the copper layers from thegraphite. Holes may be formed through the graphite, either forplated-through connectors between copper layers, on to accommodatemounting screws used to secure the board structure to a housing. At eachof the predetermined hole positions, the graphite sheets are pre-drilledand the holes are filled with a bonding adhesive material, such as theepoxy resin used to form the sheets. When via holes or mounting screwholes are subsequently drilled through the composite board, a drill ofsmaller diameter is used. In this way, each hole through a graphitelayer has an annular sheath of insulating material around it, and noinadvertent connection is made between copper layers. However, theinsulation around the mounting screws is thin enough to provide only asmall resistance to the flow of heat.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of multiple-layer printedcircuit boards. In particular, the invention provides a circuit board inwhich the coefficient of thermal expansion may be substantially reduced,and matched to that of an adjacent chip carrier. Moreover, the board ofthe invention has good thermal conductivity properties, excellentmechanical strength, and does not sacrifice dielectric performance.Other aspects and advantages of the invention will become apparent fromthe following more detailed description, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective view showing four chip carriersmounted on a circuit board;

FIG. 2 is a fragmentary cross-sectional view of a multilayer circuitboard made in accordance with the invention; and

FIG. 3 is a fragmentary view showing how via holes and mounting screwholes are formed in the board of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the drawings for purposes of illustration, the presentinvention is principally concerned with multilayer printed circuitboards. There are three desirable properties of circuit boards,especially those used to support electronic components on ceramic chipcarriers. First, the board should have a relatively low dielectricconstant, to facilitate transmission of signals over relatively longdistances. Second, the coefficent of thermal expansion of the boardshould be controllable to some degree, to match the coefficient ofadjacent materials, such as chip carriers. Finally, the board shouldideally be a sufficiently good thermal conductor to assist in thedissipation of heat generated in mounted components. Unfortunately,these three desired characteristics cannot be found in a singlematerial.

FIG. 1 shows a multilayer circuit board, indicated by reference numeral10, used to support four chip carriers 12. The chip carriers 12 haveleads brought out to their edges and extending around beneath thecarriers. The leads are then attached to the circuit board 10 by areflow soldering process in which the leads are secured to respectivepads 14 on the top of the circuit board 10. Any mismatch between thecoefficients of thermal expansion of the chip carriers 12 and the board10 can result in damage to the electrical chip connections.

In accordance with the invention, and as shown diagrammatically in FIG.2, the circuit board 10 includes a plurality of layers of graphite 16interleaved between layers 18 of a dielectric material that includespolytetrafluoroethylene (PTFE). Some of the layers 18 are copper coated,as indicated at 20. The PTFE layers 18 provide the basic dielectricmaterial of the board 10, and the graphite layers 16 provide boththermal conductivity and control of thermal coefficient of expansion.The graphite layers 16 are bonded to adjacent copper layers 20 or PTFElayers 18 with a suitable adhesive layer 22. For the copper-to-graphitebond, the adhesive 22 also functions as an electrical insulator, toavoid having the graphite act as an unwanted electrical connectionbetween portions of the copper layers. The adhesive layers 22 are,however, so thin that they offer little resistance to the flow of heatbetween the copper and the graphite.

The graphite layers 16 are formed from fibrous carbon material, such as"Thornel" P100 or P75S, made by Union Carbide Corporation, CarbonProducts Division, Chicago, Ill. 60606. The graphite or carbon yarn iswoven into a cloth, which is then impregnated with a bonding material,such as an epoxy resin. In an alternative preferred embodiment, whichincorporates high modulus carbon fibers, the carbon fibers are arrangedin each layer so that they are more or less parallel to one another andare not woven into cloth, since such weaving may result in damage to thebrittle high modulus fibers. Alternate layers are then arranged so thatthe combination of fiber directions in the layers provides the requiredregidity and strength along the major axes of the circuit board. Theresin-impregnated graphite cloth cures to a hard, regid sheet, withpractically planar surfaces. Hole locations, such as indicated at 30 inFIG. 3, are predetermined, and the graphite sheets are pre-drilled withoversized holes 32, which are immediately filled with more resinmaterial. Then the graphite boards 16 are ready to be laminated with thePTFE layers 18, using the adhesive 22.

Prior to lamination, the copper layers 20 are appropriately patterned asdictated by the design of circuits mounted on the board. Afterlamination, holes are made through the entire board 10, but using asmaller diameter drill than the one used to pre-drill the graphitelayers 16. Thus, each newly formed hole 34 is surrounded by an annularsheath of insulating material, which serves to electrically separate thegraphite layers 16 from the copper layers 20, but is thin enough topermit transmission of heat from the copper.

The number and thickness of the graphite layers 16 are selected toprovide a desired effect on the composite coefficient of thermalexpansion of the board 10. In addition, the graphite layers 16 aredisposed in a symmetrical fashion across the thickness of the board 10,to minimize the possibility of bending of the board during temperaturechanges. The coefficient of thermal expansion of the graphite layers 16is close to zero after impregnation with epoxy resin. The coefficient ofthermal expansion for copper is 9.4×10⁻⁶ in/in/°F., and the coefficientfor copper-clad PTFE sheets is much higher. The goal in selecting thenumber and thickness of the graphite layers 16 is to match thecoefficient of thermal expansion to that of the chip carrier material.The most common material, aluminum oxide, has a coefficient of thermalexpansion of 3.33×10⁻⁶ in/in/°F.

The PTFE layers 18 may be part of a suitable material, such as CU-CLAD233, manufactured by the Electronics Products Division of the 3MCompany, St. Paul, Minn. 55144. This material is a laminate of PTFE andwoven glass. It has a low dielectric constant of 2.33, but a relativelyhigh coefficient of thermal expansion. Appropriate selection andpositioning of the graphite layers 16 results in a composite coefficientof thermal expansion that closely matches that of the chip carriermaterial.

One of the principal advantages of the use of graphite in the circuitboard structure is that it serves as a good conductor of heat, whichnormally can flow from the mounted components through the copper layersof the board, through mounting bolts, and thence to a housing or otherheat sink. In the structure of the invention, the graphite layers 16provide a parallel path for the flow of heat, and thereby improve theheat dissipation characteristics of the circuit package. Anotheradvantage of the use of graphite is that it is mechanically extremelystrong. The tensile modulus of elasticity for single yarns of thegraphite specified above is in the range 75-100×10⁶ p.s.i., but this isnaturally reduced when the graphite is impregnated with resin.Nevertheless, the use of graphite greatly stengthens the circuit boardstructure.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of multilayer printedcircuit boards. In particular, the use of multiple layers of graphite ina circuit board is used to control the coefficient of thermal expansionand to provide a supplementary thermal path for the dissipation of heatfrom components mounted on the board. The base material used in theboard is PTFE with a desirably low dielectric constant.

It will also be appreciated that, although a specific embodiment of theinvention has been described in detail for purposes of ilustration,various modifications may be made without departing from the spirit andscope of the invention. Accordingly, the invention is not to be limitedexcept as by the appended claims.

I claim:
 1. A multilayer printed circuit board having a desiredcoefficient of thermal expansion, good thermal conductivity and lowdielectric constant, said circuit board comprising:a plurality of layersof conductive metal used to establish connections between components tobe mounted on the board; a plurality of layers of graphite, at leastsome of which are positioned in close proximity to some of said layersof conductive metal, to provide a relatively low resistance path for theflow of heat from said layers of conductive metal; and a plurality oflayers of a dielectric material bonded together with said layers ofconductive metal and graphite, to yield a composite multilayer printedcircuit board; wherein said layers of graphite are positioned in asymmetrical manner with respect to the thickness of the composite board,and selected in number to provide a desired composite coefficient ofthermal expansion, and formed from woven sheets of graphite impregnatedwith a bonding material; and wherein said graphite sheets are bonded toadjacent sheets of conductive metal by a thin layer of insulatingadhesive, whereby the layer of adhesive serves the additional purpose ofelectrically separating the conductive metal from the conductivegraphite layer; and wherein said sheets of conductive metal are ofcopper; and said sheets of dielectric material include apolytetrafluoroethylene (PTFE) material.
 2. A multilayer printed circuitboard having a desired coefficient of thermal expansion, good thermalconductivity and low dielectric constant, said circuit boardcomprising:a plurality of layers of conductive metal used to establishconnections between components to be mounted on the board; a pluralityof layers of graphite, at least some of which are positioned in closeproximity to some of said layers of conductive metal, to provide arelatively low resistance path for the flow of heat from said layers ofconductive metal; and a plurality of layers of a dielectric materialbonded together with said layers of conductive metal and graphite, toyield a composite multilayer printed circuit board; wherein said layersof graphite are positioned in a symmetrical manner with respect to thethickness of the composite board, and selected in number to provide adesired composite coefficient of thermal expansion; and wherein saidgraphite layers are formed from unidirectional fibers of graphiteimpregnated with a bonding material; and said graphite sheets are bondedto adjacent sheets of conductive metal by a thin layer of insulatingadhesive, whereby the layer of adhesive serves the additional purpose ofelectrically separating the conductive metal from the conductivegraphite layer.
 3. A multilayer printed circuit board as set forth inclaim 2, wherein:said sheets of conductive metal are of copper; and saidsheets of dielectric material include a polytetrafluoroethylene (PTFE)material.
 4. A multilayer printed circuit board having a desiredcoefficient of thermal expansion, good thermal conductivity and lowdielectric constant, said circuit board comprising:a plurality of layersof a dielectric material that includes polytetrafluoroethylene, some ofsaid layers being coated with patterned copper, as dictated by thenature of the components supported on the circuit board; and a pluralityof layers of graphite, at least some of which are positioned in closeproximity to some of said copper layers, to provide good thermalconduction for the dissipation of heat generated in the componentsmounted on the board; wherein said graphite layers are selected toprovide a desired composite coefficient of thermal conductivity and arepositioned across the thickness of the board in a symmetrical manner, tominimize bowing of the board in conditions of varying temperature.
 5. Aprinted circuit board as set forth in claim 4, wherein:said said layersof graphite are formed from unidirectional graphite fibers impregnatedwith a plastic bonding material.
 6. A printed circuit board as set forthin claim 5, wherein:said plastic bonding material in an epoxy resin. 7.A printed circuit board as set forth in claim 4, wherein:said layers ofgraphite are formed from woven sheets of graphite impregnated with aplastic bonding material.
 8. A printed circuit board as set forth inclaim 7, wherein:said plastic bonding material is an epoxy resin.